You can use the API provided by FreeCAD to convert a STEP file to different 3D format.
The API is written with Python, but you can use Pip - Python in PHP
Here is an example that shows how to convert a file from STEP to OBJ format using python:
import os
import ImportGui
files = os.listdir("path")
for file in files:
ImportGui.open("path" + file)
App.setActiveDocument("Unnamed")
App.ActiveDocument=App.getDocument("Unnamed")
Gui.ActiveDocument=Gui.getDocument("Unnamed")
Gui.SendMsgToActiveView("ViewFit")
__objs__=[]
__objs__.append(FreeCAD.getDocument("Unnamed").getObject("Part__Feature"))
index = 1
base = 'Part__Feature00'
while FreeCAD.getDocument("Unnamed").getObject(base + str(index)) is not None:
__objs__.append(FreeCAD.getDocument("Unnamed").getObject(base + str(index)))
index+=1
import Mesh
Mesh.export(__objs__,"path" + file + ".obj")
del __objs__
App.closeDocument("Unnamed")